Since a lithium-ion secondary battery, one type of nonaqueous electrolyte secondary batteries, is a light battery whose charging and discharging capacity is large, it has been used mainly as a battery for portable electronic devices. Moreover, lithium-ion secondary batteries have also been expected to be put into use as a battery for electrically-driven vehicles such as electric automobiles. Generally, materials compounding a rare metal such as cobalt or nickel are used as a positive-electrode active material of a lithium-ion secondary battery. However, due to the fact that rare metals are small in the distributed amount, not always easily available and additionally expensive, a positive-electrode active material using a material that replaces a rare metal has been required.
A technique of using elemental sulfur as a positive-electrode active material is known. That is, in the case where elemental sulfur is used as a positive-electrode active material, this sulfur is easily available compared to rare metals and is inexpensive, and has a further advantage that a charging and discharging capacity of a lithium-ion secondary battery can be made larger than the present state. For example, it is known that a lithium-ion secondary battery using sulfur as a positive-electrode active material can achieve about 6 times larger charging and discharging capacity than a lithium-ion secondary battery using lithium cobalt oxide which is a general positive-electrode material.
However, the lithium-ion secondary battery using elemental sulfur as the positive-electrode active material has a problem that a charging and discharging capacity is deteriorated through repeated charging and discharging. That is, elemental sulfur likely generates a compound with lithium when discharging and since the generated compound is soluble into an nonaqueous electrolyte (for example, ethylene carbonate and dimethyl carbonate and the like) of the lithium-ion secondary battery, the charging and discharging capacity is gradually reduced through repeated charging and discharging due to the sulfur eluting into the electrolyte.
Hereinafter, a property of preventing a reduction of a charging and discharging capacity associated with repeated charging and discharging is referred to as “cyclability”. It is recognized while a lithium-ion secondary battery in which a degree of reduction of a charging and discharging capacity is large and a capacity retention rate is low is inferior in cyclability, a lithium-ion secondary battery in which a degree of reduction of a charging and discharging capacity is small and a capacity retention rate is high is excellent in cyclability. In order to improve cyclability by preventing sulfur from eluting into an electrolyte, a positive-electrode active material (hereinafter also referred to as “a sulfur-based positive-electrode active material”) comprising sulfur and a material other than sulfur such as a carbon material has been proposed.
For example, Patent Document 1 discloses a technique of using a carbon polysulfide in which sulfur is bound to a carbon skeleton as a sulfur-based positive-electrode active material. A carbon polysulfide is a substance in which sulfur is added to a straight-chain unsaturated polymer, and synthesized, for example, by so-called vulcanization reaction, heat-treating polyisoprene and sulfur. By use of a sulfur-based positive-electrode active material comprising this carbon polysulfide, cyclability of a lithium-ion secondary battery can be improved to a certain degree compared to the case where elemental sulfur is used.
However, this effect is restrictive and cyclability of a lithium-ion secondary battery could not have been improved enough. It is considered that this is because sulfur and lithium are bonded at discharging, and that CS—SC bonds or S—S—S bonds in a carbon polysulfide are disconnected and then a polymer is cut.
Therefore, in order to improve cyclability, it has been considered to add polyacrylonitrile to polyisoprene (see Patent Documents 1 and 2 and the like). However, from what the inventors have studied, even if polyacrylonitrile is added, the effect obtained therefrom is restrictive and cyclability of a lithium-ion secondary battery still could not have been improved enough.
In Patent Document 3, it is described that an elution of sulfur is prevented by fixing sulfur by use of a carbon source compound comprising a polycyclic aromatic hydrocarbon in which three or more six-membered rings are fused. However, the effect obtained therefrom is also restrictive and further improvement is desired.